Retrofit of Industrial HVAC Distribution in Mumbai Textile Mill
Engineering Case Study
Case Study 2: Retrofit of Industrial HVAC Distribution in Mumbai Textile Mill
Scenario A century-old textile mill in Byramji Town, Mumbai, is upgrading its aging air handling units (AHUs) serving weaving sheds. The existing 70-year-old aluminum wiring (no insulation rating traceable) frequently trips during monsoon humidity and overheats under peak summer load. A new 3-phase, 400 V, 45 kW AHU system (two 22.5 kW units on shared feeder) must replace the legacy setup. Space in the historic concrete cable tray is severely restricted — maximum allowable cable diameter is 22 mm. The installation method is constrained to In Air (open tray, no conduit or burial), with ambient temperature ~38°C year-round and high humidity (>80% RH in monsoon). No shutdown window exceeds 4 hours — so redundancy and fault tolerance are critical.
Given Data
- Voltage: 400 V
- Power: 45,000 W (combined load of two AHUs, continuous duty)
- Power Factor: 0.78 (measured motor nameplate + VFD harmonics)
- Cable Length: 62 m (longest run from MCC to farthest AHU)
- Cable Material: Aluminum (chosen for weight, cost, and compatibility with existing lugs — copper prohibited by budget and lug re-use requirement)
- Installation Method: In Air
Calculation Using the Cable Size Calculator:
- Current calculation: ( I = \frac{45{,}000}{1.732 \times 400 \times 0.78} \approx 83.35 , \text{A} )
- Voltage drop estimation (for aluminum, in-air installation — higher convective cooling than conduit, but lower base conductivity):
- Calculator uses resistivity ρ = 28.26 nΩ·m for Al and applies IEC 60287 correction for free-air grouping and ambient.
- For 50 mm² Al in air: R ≈ 0.72 Ω/km → voltage drop ≈ ( 1.732 \times 83.35 \times 0.72 \times 0.062 \approx 6.43 , \text{V} ) (1.61% of 400 V)
- Tool outputs:
current = 83.35 A,voltage_drop = 6.43 V,cable_size = 50 mm²
- Validation check: 50 mm² Al (IS 8130) has tabulated current rating of 110 A in air at 30°C — after applying 0.82 derating for 38°C (per IS 7098-1 Table 18), rated current = 90.2 A > 83.35 A ✅. Diameter ≈ 21.4 mm — fits tray constraint.
Result and Decision The team installed 50 mm² aluminum, XLPE-insulated, steel-wire-armored (SWA) cables (IS 8130) in the existing open tray. Though 35 mm² passed the calculator’s basic current check (Irated = 85 A → 69.7 A after derating < 83.35 A ❌), it failed thermal validation. 50 mm² was the smallest size meeting all criteria: ampacity with derating, voltage drop < 3%, physical fit, and corrosion resistance (XLPE + SWA for monsoon humidity). A parallel 25 mm² pilot cable was also installed for monitoring — confirming 50 mm² reduced surface temperature by 11°C vs. legacy wiring.
Lesson
When retrofitting legacy infrastructure, the ‘minimum recommended size’ from a calculator is often the absolute floor — always cross-check physical constraints (tray fill, bend radius, termination compatibility) and apply mandatory environmental derating before final selection; skipping this step risks premature failure and unplanned downtime.